The energy distributions of beta particles which penetrated a certain matter thickness were studied experimentally and theoretically by using a Surface barrier solid state detector. A valid theoretical expression based on average values between energy and distance traveled during the slowing down of the electron wits obtained. Two analytical expressions were proposed; one for the energy distribution of monoenergetic electrons which penetrated it certain matter thickness, and one for the response function in the detector for monoenergetic electrons detected with its entire energy. Response functions of the detector for beta particles emitted from Tl-204 isotope which penetrated a certain matter thickness were obtained for two different aluminum thicknesses, and the results were discussed by comparing with experimental energy spectra. (c) 2005 Elsevier B.V. All rights reserved.